588 THE WONDER OF LIFE 



The idea which we have illustrated was clearly expressed 

 by Dr. Anton Dohrn, the founder of the famous Zoological 

 Station at Naples. He called it ' the principle of function- 

 change ', and showed, for instance, that the unimportant 

 bladder which grows out from the hind end of the gut in 

 frogs, becomes an all-important birth- robe, the allantois 

 in reptiles and birds, and part of the placenta, which 

 binds the unborn young to the mother, in mammals. 



New things out of old, that is the law. For what could 

 have been newer in its day than a feather, and what is a 

 feather but a glorified scale ? In regard to this homology, 

 which Aristotle discerned so long ago, there is still a little 

 difficulty, for the development of a feather is very distinc- 

 tive and in several respects unlike that of a scale. And 

 there are no transitional types between scale and feather, 

 the minute flat structures on a penguin's flippers being no 

 nearer scales than are the plumes of an eagle. Recent 

 investigations, such as Frieda Bornstein's study of the 

 foot of the capercaillie, where feathers and scales occur in 

 close association, point to the conclusion that a feather 

 corresponds not to a whole scale, but to part of a scale, 

 another part being suppressed. But none the less the 

 feather illustrates the evolution of the new from the old. 



As another illustration of tactics we would briefly refer 

 to the idea of temporal variations which has been expounded 

 by Professor Patrick Geddes. In the chapter on ' The 

 Cycle of Life ' we have spoken of the changes which may 

 come about by lengthening out one chapter of the life- history 

 and compressing another, by altering, as it were, ' the time ' 

 of the tune at different periods. It seems that we can 

 interpret not a few evolutionary changes in the light 

 of this idea. For some types are all youth, and others 



